Methods of preparing alkylene carbonates are known in the art, however the methods used in the past generally involved rather indirect routes and expensive reactants and often employed reaction mechanisms which were susceptible to steric hindrances.
U.S. Pat. No. 2,773,070 describes one of the earlier methods of preparing alkylene carbonates which comprises reacting an alkylene oxide with a molar excess of carbon dioxide at a temperature between 100.degree. C. and 225.degree. C. and a pressure in excess of 300 psig in the presence of a catalyst comprising one of a specified group of ammonium halides.
Early art in the field indicates that cyclic carbonate esters of 1,2-diols can be reacted with thiocyanate salts in the synthesis of episulfides, however the reaction was found to be quite susceptible to steric hindrance. See J. Org. Chem. (1962), 27, 2832.
An article in J. Am. Chem. Soc. (1962) 84, 747 discusses, among other things, the high-resolution proton nuclear magnetic resonance spectra which were determined for the isomers of the cyclic carbonate of 2,3-butanediol. Chemical shifts and coupling constants for the compounds are shown.
There is disclosed in U.S. Pat. No. 3,025,305 a process for the production of cyclic carbonates which comprises reacting a monoolefin of about 2 to about 30 carbon atoms with carbon dioxide having a partial pressure of at least about 500 psig and a molecular oxygen-containing gas at a temperature of about 200.degree. to 400.degree. F. and a total pressure sufficient to maintain the liquid phase using two catalysts, a cobalt organic salt and a type of quaternary ammonium compound.
In U.S. Pat. No. 3,923,842 there is disclosed a process for the preparation of an oxirane compound from the corresponding olefin. Here a vicinal halohydrin is formed by reacting the corresponding olefin with oxygen in the presence of an iron halide and a copper halide, under reaction conditions where iron oxide is formed as a coproduct, and is reacted with an amine and carbon dioxide to form one of a group of identified cyclic carbonate esters.
In U.S. Pat. No. 4,009,183 there is disclosed a process for preparing alkylene carbonates from olefins reacted with carbon dioxide in the presence of iodine or certain iodine-containing compounds and an oxygen conveyor at a temperature between 30.degree. and 120.degree. C. and at a pressure between atmospheric to 100 atmospheres and a pH value between 3 and 8.
In U.S. Pat. No. 4,224,223 there is described a process for the preparation of a cyclic alkylene carbonate ester which comprises reacting a cyclic or linear olefin having from 2 to 15 carbon atoms in liquid phase in the presence of oxygen or an oxygen-containing gas and a catalytic amount of an iodine or iodide of a metal and a catalytic iron or copper compound or mixture thereof with carbon dioxide at a temperature of from 50.degree. to 160.degree. C. at a total pressure of from 200 to about 2000 psig and a pH value of between about 4 and 8.
Venturello and D'Aloisio have described a method of preparing 1,2-alkanediyl carbonates in high yields in short reaction times under mild conditions without the drawbacks of toxic or hazardous reagents or high temperatures or the formation of undesired glycols which are hard to separate from cyclic carbonates. This method comprises stirring the corresponding vic-halohydrins with tetramethyl ammonium hydrogen carbonate in acetonitrile. See "A Convenient Synthesis of 1,2-Alkanediyl Carbonates." Synthesis (1985), 1, 33.
In DE 3723782C (Dainippon Ink Chem KK) cyclocarbonates are prepared from a vicinal halo-hydrin and alkali bicarbonates by heating in aprotic organic solvents such as dimethyl sulfoxide, acetonitrile and dimethylformamide.
From the available art it does not appear that any skilled in the art have heretofore considered a method for preparing alkylene carbonates, especially 1,2-butylene carbonate, from the corresponding alkylene glycol and urea. In the instant invention it has been surprisingly discovered that 1,2-butanediol and urea can be reacted in the presence of, optionally, a tin catalyst to prepare 1,2-butylene carbonate. This process should constitute a very desirable advance in the field because it is comparatively simple, the starting materials are relatively cheap and mild conditions can be employed.